Along with the utilization of an optical communication system, a large capacity and multi-function system is desired. Also, enhanced functions such as high-speed generation of optical signal, high-speed switching of optical transmission lines, and high-speed exchanging are required.
As a means for switching optical transmission lines and exchanging in a network, an optical switch is used. The optical switch now available is operated by switching light paths according to the mechanical movement of a prism, mirror, fiber or the like. However, it has problems that the operating speed is low and that it is too large to construct a matrix switch. To overcome the problems, a waveguide-type optical switch which employs optical waveguides has been developed. It has advantages in that the high-speed operating, integration of numerous elements and high reliability can be realized. In particular, the optical switch which employs a ferroelectric material such as lithium niobate (LiNbO.sub.3) or the like has low light absorption and high efficiency caused by the large electrooptic effect. Various types of optical control devices are reported, for example, a directional-coupler type, mach-zender type, balance-bridge type, total internal reflection type optical switch.
Recently, the high-density integration of the waveguide-type optical switch using a directional coupler which is formed in an electrooptic effect LiNbO.sub.3 crystalline substrate has been developed. H. Nishimoto (the inventor of this application) et al., "Polarization Independent 8.times.8 LiNbO.sub.3 Optical Matrix Switch", Electronic Information Communication Society, OQE88-147, pp.67-74 reports the 8.times.8 matrix optical switch in which 64 directional-coupler type optical switches is integrated in the LiNbO.sub.3 crystalline substrate. Also, a device such as an external optical modulator which comprises a single optical switch has been developed.
The characteristics concerning such waveguide-type devices are stability in operation, switching voltage (power), crosstalk, extinction ratio, loss, switching speed and so on. Most important points of these characteristics are stability in operation and reduction in switching voltage (power).
A conventional optical control device has in general a structure in which a directional coupler consisting of two optical waveguides are formed in a LiNbO.sub.3 or LiTiO.sub.3 substrate which has a pyroelectric effect.
However, in such optical control device, there is a problem in reliability that the localization of an electric charge generated with temperature change due to the pyroelectric effect causes the voltage change of operating point, i.e., temperature drift.
To overcome the problem, another conventional optical control device is suggested(Japanese patent application laid-open Nos. 62-73207, 62-173428). In such device, the conduction between a pair of electrodes is provided by conductive film, whereby the electric charge generated with temperature change is uniformed to suppress the temperature drift.
However, in this device, it is required that the resistance value between the electrodes through the conductive film be 10.sup.7 to 10.sup.10 .OMEGA.. Low resistance value is defined to avoid the operation error caused by leak current between the electrodes. Upper resistance value is determined to suppress the temperature drift. Thus, it requires a strict temperature management in thermal treatment process to control the resistance value. As a result, the process is complicated and the product yield is reduced.
Furthermore, since the electrodes are electrically conducted, a carrier such as an impurity may easily move inside and at the surface of the conductive film and at the interface between the conductive film and a contacting layer thereto when an external potential difference is applied between the electrodes to operate the device. When the carrier movement occurs, the external applied voltage will be canceled between the electrodes. Namely, in the above device, there is also a problem in reliability that optical output-applied voltage characteristics shifts when a DC voltage is continuously applied, i.e., DC drifting.
On the other hand, a conventional optical control device, in which the operating voltage is reduced when the conduction between electrodes is provided by conductive film is suggested (Japanese patent application laid-open No. 1-302325).
In this device, a buffer layer is formed only on two optical waveguides and is covered with conductive film, a potential difference is applied between the electrodes through the conductive film.
In this structure, the potential difference generated between the two optical waveguides is nearly equal to a value obtained by subtracting two vertical voltage drops from the external applied voltage. It is apparent that the vertical and horizontal voltage drops are simply determined by the ratio of vertical and horizontal lengths of the conductive film between the electrodes.
Accordingly, the potential difference generated between the optical waveguides in this structure is not so different from that in the other conventional devices. Therefore, the operating voltage is not so reduced.